A geophone is a device for converting ground movement (displacement) into an electrical signal, which may be recorded at a recording station. The deviation of this measured signal from the base line is called the seismic response and is analyzed to determine structure of the earth. Geophones are used by seismologists to study the earth and are also used in oil and gas exploration to map underground structures and locate oil and gas deposits.
Geophones may also be used for other purposes, including alarm systems and military applications, where ground motion may detect movement of people or vehicles. In addition, geophones have a very broad market in data acquisition technology. The accelerometers used in earth movement monitoring are large 0 to 10 Hz geophones and are distributed all over the globe in autonomous-nodal stations to study and forewarn of disastrous earth movements. Smaller versions are required in buildings in areas like Japan to study the infrastructure when earthquakes and aftershocks occur. There are many placed per floor and permanently deployed. Large machinery like turbines may require geophones to monitor the bearings, armatures, and the like for cracks/wear and for proactive maintenance.
Geophones have historically been analog devices and originally comprised a spring-mounted magnetic mass moving within a wire coil to generate an electrical signal. In more recent times, geophones have utilized a wire coil connected to a spring or springs, which allow the coil to move over a stationary magnet. Geophones are based on an inertial mass (proof mass) suspended from a spring. They function much like a microphone or loudspeaker, with a magnet surrounded by a coil of wire. In modern geophones, the magnet is fixed to the geophone case, and the coil represents the proof mass.
The frequency response of a geophone is that of a damped sinusoid, mainly determined by corner frequency (typically around 10 Hz) and damping (typically 0.707). Because the corner frequency is proportional to the inverse root of the moving mass, geophones with low corner frequencies (<1 Hz) become impractical. It is possible to lower the corner frequency electronically at the price of higher noise and cost.
Traditionally, geophones have been passive analog devices, outputting a low-voltage, (e.g., substantially 3 V) low-current (e.g., substantially 3.3 mA or less) analog signal, which requires amplification as well as conversion into digital form using an analog-to-digital converter (ADC). Traditional geophone sensors measured voltage at the geophone, rather than current. As the signal is a very low power signal, it may tend to be noisy and difficult to measure, as the sensor needs to detect small variations in voltage. In a traditional ADC solution, an amplifier amplifies the output voltage of the geophone. The amplified output is then digitized by a high-resolution ADC. The amplifier is needed to sense the geophone signals, as the signals are weak and noisy. A parallel resistor may provide for a known load. However, such amplifiers may require additional power, which may not be available, particularly for battery-operated and remote installations.
U.S. Patent Publication No. 2004/0252585 to Smith dated Dec. 16, 2004 (hereafter referred to as “Smith”) and incorporated herein by reference, discloses the benefits of a digital geophone system. Smith notes that one problem associated with traditional geophone systems is noise immunity. For longer distances, the geophone signal must be amplified and re-transmitted. This concatenation of cable and amplifiers adds system noise to the original seismic signal, thus decreasing the overall signal to noise ratio of the geophone. Smith attempts to overcome these problems by using a digital geophone. However, one problem with this approach is the power required to support the proposed digital geophone of Smith.
U.S. Pat. No. 7,518,954 to Hagedoorn issued Apr. 14, 2009 (hereafter referred to as “Hagedoorn”) and incorporated herein by reference, discloses a geophone with an internal cavity that houses an electronic circuit, mainly to accommodate an amplifier plus other electronics. As illustrated in the figures of Hagedoorn, it can be seen that the geophone transducers were mainly used in machine health type application. In some of these applications, a power cable is required to supply the electronics, but it is not the ideal situation. To mount a transducer on a rotating armature makes it hard to connect a wire, so in other applications, a battery (and some means to recharge) is applied. For battery-powered geophone applications, lower power consumption is desirable.
The limitations of the moving coil geophone recording system are largely in the data acquisition electronics and traditionally, the acceleration signal created by a geophone is converted to a voltage. Measuring voltage was beneficial when building an array of geophones in order to cancel the ground roll and to sum (amplify) reflected source signals.
A publication entitled “Seismic data recording: the limiting component” by A. S. Badger, A. D. Beecroft, A. Stienstra, SEG conference (1990), and incorporated herein by reference, disclosed that the geophone dynamic range is limited at one end by physics (Brownian motion) and by mechanics at the other (the spring may add a large horizontal signal which distorts the overall signal). Therefore, the dynamic range of a moving coil device without a damping resistor is ˜140 dB. When used in the voltage mode, the damping resistor is typically in the range of 1,000-10,000 ohms.
Also, the geophone collects data from its resonant frequency to the point where mechanical limitations start to set in. Therefore, by removing a damping resistor, the benefits may be twofold. One benefit is that it may eliminate certain thermal (Johnson) noise, and another benefit is that it may control the resonant frequency via the feedback current. This control allows the moving coil sensor to lower the bandwidth to 1-2 Hz, which expands the recordable bandwidth and obtains the sub-10 Hz signal that the oil exploration industry may be interested in. It is believed that Schlumberger, among others, have implemented the moving coil geophone accelerometer and have proven its benefits. One problem with these systems, however, involves acquiring data at ultra-low power while maintaining the fidelity of the sensor. By controlling the resonant frequency, one may be able to record down to 1-2 Hz, and one can increase the spring tension to build a 28 Hz geophone, increasing the fidelity at higher frequency and allowing the geophone to be put, for example, in a shear wave topology.
Electrical current produced by a moving coil geophone is very small (e.g., substantially 3.3 mA or less) and thus needs to be amplified in traditional data acquisition circuits. In traditional data acquisition circuits, a programmable gain amplifier is used typically to amplify the geophone signals. The programmable gain amplifier demands power from the power supply, which in a geophone installation could be a battery or a long cable. It would be desirable to eliminate the programmable gain amplifier from a geophone instrumentation design in order to reduce power consumption. It would also be desirable to address one or more other shortcomings referenced here, each of which is by way of example only.